The process of fermenting drinks intentionally has been recorded as early as the late Stone Age. Considering the different methods and resources used around the world is safe to assume these ancient people understood they were making things alcoholic. (Patrick, Clarence Hodges, 1952 Alcohol, Culture, and Society) (Chrzan, Janet ,2013 Alcohol: Social Drinking in Cultural Context)

Regarding how long we understand that alchohol is a chemical element present in feremented drinks the credits are normally given to two Arabs from the Islamic Golden Age Muhammad ibn Zakariya al-Razi (801–873 AD) and Abu Yūsuf Yaʻqūb ibn ʼIsḥāq aṣ-Ṣabbāḥ al-Kindī (854–925 AD).

Al-Kandi is the author of "The Book of the chemistry of Perfume and Distillations", which contains a detailed description of the destilation of wine.

However, Al-Razi (also know as Rasis or Rhazes) has probably the best claim as his works move further away from alchemy and potions. He is known for describing a very effective method of destilation to obtain alcohol as well as separating it. (Stefan Schlosser, 2012, Distillation – from Bronze Age till today) (Modanlou, H. D., 2008, A tribute to Zakariya Razi - an Iranian pioneer scholar)

It is possible that someone else understood and studied the process before but since there are no surviving records these two normally get the credit.